In order to handle and connect a semiconductor die (integrated circuit device) to external systems, it is generally necessary to package the die. This usually involves mounting the die to some sort of substrate, leadframe or carrier, connecting bond pads on the die to some sort of conductive lines or traces and forming a package body around the die. The conductive lines or traces exit the package body, and usually terminate in external leads or pins.
For example, ceramic packages have a package body with a central opening (cavity) in one face for receiving the die, and lead fingers embedded in the body and extending into the opening. The die is connected (usually wire bonded) to the exposed (in the opening) portions of the lead fingers. The lead fingers are connected (internally in the package) to pins exiting a planar surface of the package. These pins are typically arranged in a rectangular (e.g., square) array ("pin grid array"). In some instances, the die-receiving cavity is "up", on one face of the package body, and the pins are on the other, opposite face of the package body. In other instances, the die-receiving cavity is "down", on the same face of the package as the pins (in which case there are no pins in the area of the cavity). (The pins are deemed to be on the "bottom" of the ceramic body.)
Heat is inevitably generated during operation of a semiconductor device, and may become destructive of the device if left unabated. Therefore, it is generally well known to provide some sort of heat sink for such devices. Generally, heat sinks take one of two forms. They may be integral with the device package or they may be external to the device package. In either case, heat sinks generally include a thermal mass in intimate heat conductive relationship to the semiconductor device, and may involve air convection or forced air cooling of the thermal mass.
External heat sinks are mounted in some manner, such as with an adhesive (e.g., silver epoxy), to the semiconductor package which may be provided with thermal slugs and the like to ensure that heat is transferred from the semiconductor die (device) to the heat sink. Evidently, if the heat sink were to become un-adhered, and fall off of the package, during subsequent operation of the device, the device would be likely to become overheated and fail. This is entirely unacceptable.
Hence, it is known to test the integrity of the heat sink mount, to ensure that the heat sink remains securely mounted to the semiconductor package throughout the useful lifetime of the semiconductor device.
FIG. 1A shows a typical, cavity-down, ceramic, pin grid array semiconductor package 100, with an external heat sink adhered thereto. A semiconductor die (not visible) is contained within a ceramic package body 102. Pins 104 extend from the bottom 102b of the body. A cavity (not visible) is provided on the bottom of the body for receiving the die. The top surface 102a of the package body is essentially a flat planar surface. Thermal plugs 106 may be provided extending into the package body from the top surface to a die attach pad (not visible) within the package body to which the die is mounted, to ensure a good thermal path from the die to the top (exterior) surface of the package body. More particularly, there is a "heat sink receiving" area 108 (shown as dashed lines) defined on the top surface of the package, and the thermal plugs (if any) would be located within this area. The heat sink receiving area is flush with the top surface of the package, and is intended to receive a heat sink.
An external heat sink 110 is provided (shown spaced apart from the package body, and in cross-section). The heat sink is typically a generally cylindrical structure, having a number of disc-like radially-extending fins 112 spaced along its axis, and a flat bottom surface 110b. Notably, there is a gap (space) 114 between adjacent fins 112. An adhesive (not shown) is provided between the flat bottom surface 110b of the heat sink and the heat sink receiving area of the package body. The heat sink is assembled to the area 108 of the package with the adhesive. The type and amount of adhesive, curing techniques, and the like, are critical process parameters affecting the ultimate structural integrity of the heat sink mount.
FIG. 1B shows another type of semiconductor package 100', such as a plastic package, wherein the top surface 120a of the package body 120 is provided with a thermally-conductive slug 126 extending above the top surface 120a and defining the heat sink receiving area (compare 108). A heat sink, such as the heat sink 110 would be adhered to the top surface 126a of the slug 126.
FIG. 2A illustrates a prior art technique 200 for testing the mounting integrity of a heat sink, such as the heat sink 110 of FIG. 1A. (The technique is equally applicable to the packages of FIGS. 1A and 1B.) A "generic" package body 202 has a heat sink 210 mounted by a thin layer of adhesive 216 to the top surface of the package body. (Pins, and the like, are omitted from the view for illustrative clarity.) The heat sink 210 is placed in a recessed fixture (not shown), and a shear force indicated by an arrow 220 is applied to the package body along an axis 222. This may be done manually by an operator, with a torque wrench adapted to grasp the package body and apply a shearing force between the heat sink and the package body. This method has proven to be manually difficult to perform, to yield inconsistent and un-precise results, and can present a danger to the operator. (When the adhesive bond between the heat sink and the package body fails, there is a sudden "release" of components in close proximity to the operator.) Moreover, this method is only suitable for use with relatively large heat sinks.
FIG. 2B illustrates another technique 200' for testing the mounting integrity of a heat sink on a semiconductor package. In this case, the package body is held by hand, and the tip 232 of a torque wrench 230, is inserted by the operator between the heat sink and the package. The operator manually applies a force, indicated by the arrow 220', sufficient to separate the heat sink from the body (or vice-versa). This suffers from many of the disadvantages of the torque wrench method 200. Also this method is only useful for one type of heat sink.
What is needed is a reliable, safe, quantifiable technique for characterizing the mounting integrity of the adhesive bond between a heat sink and a semiconductor package body.